Long-term high temperature and humidity stable holographic optical data storage media compositions with exceptional high dynamic range

ABSTRACT

A novel photoimageable system for a two-chemistry system containing liquid photoreactive asymmetric acrylate compound containing sulfur, aromatic moieties, and optionally bromine, and an aluminum salt compound is disclosed. The photoimageable system has high dynamic range (M/#) and sensitivity and unexpectedly high temperature and high humidity resistance. The photoimageable system retains its dynamic range when exposed to 60° C. for 4 weeks while a composition without the aluminum salt compound lost 75% of its dynamic range under similar exposure conditions. In one embodiment, 2-10 weight % of a thio-butylacrylate dissolved in a two-component urethane matrix containing 0.002 to 1 weight % of the aluminum salt compound showed a dynamic range of greater than 5 for a 200 microns thick sample and retained more than 80% of the dynamic range after 4 weeks exposure at 60° C.

Related Application

This application claims priority from U.S. Provisional Application60/383,608, filed May 29, 2002, which is entitled the same as thisapplication and is incorporated herein by reference.

Field of the Invention

The invention relates to optically clear holographic data storage mediacompositions that have exceptional dynamic range, high sensitivity, noinhibition time, and low shrinkage. The invention also relates to saidcompositions that have long-term storage and archival life stabilitiesunder high temperature and humidity conditions. The use of the compoundsthus includes holographic optical data storage, optical lenses, beamsteerers, and waveguides.

Background

Developers of information storage devices and methods continue to seekincreased storage capacity. As parts of this development, so-calledpage-wise memory systems, in particular holographic systems, have beensuggested as alternatives to conventional memory devices.

In the typical holographic storage system, two coherent light beams aredirected onto a storage medium. The first coherent light beam is asignal beam, which is used to encode data. The second coherent lightbeam is a reference light beam. The two coherent light beams intersectwithin the storage medium to produce an interference pattern. Thestorage medium records this interference pattern by changing its indexof refraction to form an image of the interference pattern.

The recorded information, stored as a holographic image, can be read byilluminating the holographic image with a reference beam. When theholographic image is illuminated with a reference beam at an appropriateangle, a signal beam containing the information stored is produced. Mostoften the appropriate angle for illuminating the holographic image willbe the same as the angle of the reference beam used for recording theholographic image. More than one holographic image may be stored in thesame volume by, for example, varying the angle of the reference beamduring recording.

The capabilities of holographic storage systems are limited in part bythe storage media. Iron-doped lithium niobate has been used as a storagemedium for research purposes for many years. However, lithium niobate isexpensive, exhibits poor sensitivity (1 J/cm²), has low index contrast(Δn of about 10⁻⁴), and exhibits destructive read-out (i.e., images aredestroyed upon reading). Alternatives have therefore been sought,particularly in the area of photosensitive polymer films. See, e.g., W.K. Smothers et al., “Photopolymers for Holography,” SPIE OE/LaserConference, 1212-03, Los Angeles, Calif., 1990. The material describedin this article contains a photoimageable system containing a liquidmonomer material (the photoactive monomer) and a photoinitiator (whichpromotes the polymerization of the monomer upon exposure to light),where the photoimageable system is in an organic polymer host matrixthat is substantially inert to the exposure light. During writing ofinformation into the material (by passing recording light through anarray representing data), the monomer polymerizes in the exposedregions. Due to the lowering of the monomer concentration caused by thepolymerization, monomer from the dark, unexposed regions of the materialdiffuses to the exposed regions. The polymerization and resultingconcentration gradient create a refractive index change, forming thehologram representing the data. Unfortunately, deposition onto asubstrate of the pre-formed matrix material containing thephotoimageable system requires use of solvent, and the thickness of thematerial is therefore limited, e.g., to no more than about 150 μm, toallow enough evaporation of the solvent to attain a stable material andreduce void formation. In holographic processes such as described above,which utilize three-dimensional space of a medium, the storage capacityis proportional to a medium's thickness. Thus, the need for solventremoval inhibits the storage capacity of a medium. (Holography of thistype is typically referred to as volume holography because a Klein-CookQ parameter greater than 1 is exhibited (see W. Klein and B. Cook,“Unified approach to ultrasonic light diffraction,” IEEE Transaction onSonics and Ultrasonics, SU-14, 1967, at 123-134). In volume holography,the media thickness is generally greater than the fringe spacing,)

U.S. Pat. No. 6,013,454 and application Ser. No. 08/698,142, thedisclosures of which are hereby incorporated by reference, also relateto a photoimageable system in an organic polymer matrix. In particular,the application discloses a recording medium formed by polymerizingmatrix material in situ from a fluid mixture of organic oligomer matrixprecursor and a photoimageable system. A similar type of system, butwhich does not incorporate oligomers, is discussed in D. J. Lougnot etal., Pure and Appl. Optics, 2, 383 (1993). Because little or no solventis typically required for deposition of these matrix materials, greaterthicknesses are possible, e.g., 200 μm and above. However, while usefulresults are obtained by such processes, the possibility exists forreaction between the precursors to the matrix polymer and thephotoactive monomer. Such reaction would reduce the refractive indexcontrast between the matrix and the polymerized photoactive monomer,thereby affecting to an extent the strength of the stored hologram.

Thus, while progress has been made in fabricating photorecording mediasuitable for use in holographic storage systems, further progress isdesirable. In particular, compositions that have exceptional highdynamic ranges are highly desired along with their abilities to maintainhigh performances for long-term in high temperature and high humidityenvironment. In addition, rapid production of high performanceholographic media which are capable of being formed in relatively thicklayers, e.g., greater than 200 μm, which substantially avoid reactionbetween the matrix material and photoactive monomers, and which exhibituseful holographic properties, are also desired.

SUMMARY OF THE INVENTION

This invention constitutes an improvement over prior recording media.New compositions having higher M/#, high sensitivity, and low shrinkageare described. In addition, said compositions can maintain higherperformance characteristics when subject to long-term exposure to hightemperature and humidity environment.

Holographic optical media compositions for data recording at 532-nmrange employ a photoinitiator/oxidizer system to provide optically clearproducts for high performance data storage applications. The presence ofoxidizers cause instability of the media, for example, slow decay of theM/# at ambient and elevated temperatures, and poor resistance to highhumidity. Commonly known thermal stabilizers had found to have limitedeffectiveness in stabilizing the M/#. In addition, these thermalstabilizers impart undesirable inhibition times and reduced sensitivityof the media.

It has been found that an aluminum salt compound, used by itself or incombination with other thermal stabilizers, provide the desiredstability at high temperatures and humidity.

Preferably, the aluminum salt compound has the following formula:[X−R−N(NO)O]_(n)M²where, X=H, CH₃, OCH₃, F, Cl, CF₃ or SOCH₃; R is an aliphatic, alicyclicor aromatic group preferably having 1-18 carbon atoms; n=0-5; M² ishydrogen, a group I to III metal, a group VIIIB metal, or a substitutedor unsubstituted NH4 group. Preferably M² is Al³⁺ where n=3, and M² isNH4+where n=0. Surprisingly, this aluminum salt compound had eliminatedthe inhibition times while maintained other desired high performancecharacteristics, such as high M/#, high sensitivity and low shrinkage ofthe media.

This invention also describes novel photoactive compounds which provideexceptionally high M/#, high sensitivity, and low shrinkage whenincorporated in an independent polymeric matrix holographic data storagemedia system. The instant photoactive compounds can be represented bythe following chemical structure.

where R can be phenyl, mono or multi-substituted phenyls, bromophenyl,or naphthalene group; R₁ and R₃ can be methylene, ethylene, propylene,or butylene group; R₂ can be H, or alkyl group, and R₃ can be COCH═CH₂,COCCH₃═CH₂, or CH═CH₂. Synthesis, properties, and applications of theabove compounds will be illustrated herein.

The M/# is defined to be the dynamic range of the recording material.The M/# is measured by multiplexing a series of holograms with exposuretimes set to consume all of the photoactive materials in the media. TheM/# is then the sum of the square roots of the diffraction efficienciesof all of the multiplexed holograms. The M/# depends on the thickness ofthe media.

The sensitivity is measured by the cumulative exposure time required toreach 80% of the total M/# of the recording medium. The higher thesensitivity of the material, the shorter the cumulative exposure timerequired to reach 80% of the total M/#.

The shrinkage (occurring primarily in the thickness of the medium) isdetermined by measuring the Bragg detuning (the shift in the readoutangle) of the angle multiplexed holograms. The quantitative relationshipbetween the physical shrinkage of the material and the Bragg detuning isdescribed in detail in the above reference, i.e., Applied PhysicsLetters, Volume 73, Number 10, p. 1337-1339, 7 Sep. 1998.

The inhibition time is defined as the time it takes for the holograms toform from the time the media is exposed to a light source.

DETAILED DESCRIPTION

The optical article, e.g., holographic data recording medium, of theinvention is formed by steps including mixing a matrix precursor and aphotoactive monomer of a photoimageable system, and curing thephotoimageable system to form the matrix in situ. The matrix precursorand photoactive monomer are selected such that the following conditionsare preferentially met by such the photoimageable system. (1) Thephotoimageable system is a “two-chemistry system” such that the reactionby which the matrix precursor is polymerized during the cure isindependent from the reaction by which the photoactive monomer will bepolymerized during writing of a pattern, e.g., data. (2) The matrixpolymer and the polymer resulting from polymerization of the photoactivemonomer (the photopolymer) are compatible with each other. As discussedpreviously, the matrix is considered to be formed when thephotorecording material, i.e., the matrix material plus the photoactivemonomer, photoinitiator, and/or other additives, exhibits an elasticmodulus of at least about 10⁵ Pa, generally about 10⁵ Pa to about 10⁹Pa, advantageously about 10⁶ Pa to about 10⁸ Pa.

The compatibility of the matrix polymer and photopolymer tends toprevent large-scale (>100 nm) phase separation of the components, suchlarge-scale phase separation typically leading to undesirable hazinessor opacity. Utilization of a photoactive monomer and a matrix precursorthat polymerize by independent reactions provides a cured matrixsubstantially free of cross-reaction, i.e., the photoactive monomerremains substantially inert during the matrix cure. In addition, due tothe independent reactions, there is no inhibition of subsequentpolymerization of the photoactive monomer. At least one photoactivemonomer contains one or more moieties, excluding the monomer functionalgroups, that are substantially absent from the polymer matrix, i.e., itis possible to find a moiety in the photoactive monomer such that nomore than 20% of all such moieties in the photorecording material arepresent, i.e., covalently bonded, in the matrix. The resulting opticalarticle is capable of exhibiting desirable refractive index contrast dueto the independence of the matrix from the photoactive monomer.

As discussed above, formation of a hologram, waveguide, or other opticalarticle relies on a refractive index contrast (An) between exposed andunexposed regions of a medium, this contrast at least partly due tomonomer diffusion to exposed regions. High index contrast is desiredbecause it provides improved signal strength when reading a hologram,and provides efficient confinement of an optical wave in a waveguide.One way to provide high index contrast in the invention is to use aphotoactive monomer having moieties (referred to as index-contrastingmoieties) that are substantially absent from the matrix, and thatexhibit a refractive index substantially different from the indexexhibited by the bulk of the matrix. For example, high contrast would beobtained by using a matrix that contains primarily aliphatic orsaturated alicyclic moieties with a low concentration of heavy atoms andconjugated double bonds (providing low index) and a photoactive monomermade up primarily of aromatic or similar high-index moieties.

Preferred photoactive monomers can be represented by the followingchemical structure.

where R can be phenyl, mono or multi-substituted phenyls, bromophenyl,or naphthalene group; R₁ and R₃ can be methylene, ethylene, propylene,or butylene group; R₂ can be H, or alkyl group; and R₃ can be COCH═CH₂,COCCH₃═CH₂, or CH═CH₂.

The matrix is a solid polymer formed in situ from a matrix precursor bya curing step (curing indicating a step of inducing reaction of theprecursor to form the polymeric matrix). It is possible for theprecursor to be one or more monomers, one or more oligomers, or amixture of monomer and oligomer. In addition, it is possible for theprecursor to be greater than one type of precursor functional groups,either on a single precursor molecule or in a group of precursormolecules. (Precursor functional groups are the group or groups on aprecursor molecule that are the reaction sites for polymerization duringmatrix cure.) To promote mixing with the photoactive monomer, theprecursor is advantageously liquid at some temperature between about−50° C. and about 80° C. Advantageously, the matrix polymerization iscapable of being performed at room temperature. Also advantageously, thepolymerization is capable of being performed in a time period less than5 minutes. The glass transition temperature (T_(g)) of thephotorecording material is advantageously low enough to permitsufficient diffusion and chemical reaction of the photoactive monomerduring a holographic recording process. Generally, the T_(g) is not morethan 50° C. above the temperature at which holographic recording isperformed, which, for typical holographic recording, means a T_(g)between about 80° C. and about −130° C. (as measured by conventionalmethods).

Examples of polymerization reactions contemplated for forming matrixpolymers in the invention include isocyanate-hydroxyl steppolymerization (urethane formation), isocyanatae-amine steppolymerization (urea formation), cationic epoxy polymerization, cationicvinyl ether polymerization, cationic alkenyl ether polymerization,cationic allene ether polymerization, cationic ketene acetalpolymerization, epoxy-amine step polymerization, epoxy-mercaptan steppolymerization, unsaturated ester-amine step polymerization (via Michaeladdition), unsaturated ester-mercaptan step polymerization (via Michaeladdition), and vinyl- silicon hydride step polymerization(hydrosilylation).

Several such reactions are enabled or accelerated by suitable catalysts.For example, cationic epoxy polymerization takes place rapidly at roomtemperature by use of BF₃-based catalysts, other cationicpolymerizations proceed in the presence of protons, epoxy-mercaptanreactions and Michael additions are accelerated by bases such as amines,hydrosilylation proceeds rapidly in the presence of transition metalcatalysts such as platinum, and urethane and urea formation proceedrapidly when tin catalysts are employed. It is also possible to usephotogenerated catalysts for matrix formation, provided that steps aretaken to prevent polymerization of the photoactive monomer during thephotogeneration.

The photoactive monomer is any monomer or monomers capable of undergoingphotoinitiated polymerization, and which, in combination with a matrixmaterial, meets the polymerization reaction and compatibilityrequirements of the invention. Suitable photoactive monomers includethose which polymerize by a free-radical reaction, e.g., moleculescontaining ethylenic unsaturation such as acrylates, methacrylates,acrylamides, methacrylamides, styrene, substituted styrenes, vinylnaphthalene, substituted vinyl naphthalenes, and other vinylderivatives. Free-radical copolymerizable pair systems such as vinylether mixed with maleate and thiol mixed with olefin are also suitable.It is also possible to use cationically polymerizable systems such asvinyl ethers, alkenyl ethers, allene ethers, ketene acetals, andepoxies. It is also possible for a single photoactive monomer moleculeto contain more than one monomer functional group. These monomers couldused as by themselves or in combination in a mixture.

As mentioned previously, relatively high index contrast is desired inthe article of the invention, whether for improved readout in arecording media or efficient light confinement in a waveguide. Inaddition, it is advantageous to induce this relatively large indexchange with a small number of monomer functional groups, becausepolymerization of the monomer generally induces shrinkage in a material.

Such shrinkage has a detrimental effect on the retrieval of data fromstored holograms, and also degrades the performance of waveguide devicessuch as by increased transmission losses or other performancedeviations. Lowering the number of monomer functional groups that mustbe polymerized to attain the necessary index contrast is thereforedesirable. This lowering is possible by increasing the ratio of themolecular volume of the monomers to the number of monomer functionalgroups on the monomers. This increase is attainable by incorporatinginto a monomer larger index-contrasting moieties and/or a larger numberof index-contrasting moieties. For example, if the matrix is composedprimarily of aliphatic or other low index moieties and the monomer is ahigher index species where the higher index is imparted by a benzenering, the molecular volume could be increased relative to the number ofmonomer functional groups by incorporating a naphthalene ring instead ofa benzene ring (the naphthalene having a larger volume), or byincorporating one or more additional benzene rings, without increasingthe number of monomer functional groups. In this manner, polymerizationof a given volume fraction of the monomers with the larger molecularvolume/monomer functional group ratio would require polymerization ofless monomer functional groups, thereby inducing less shrinkage. But therequisite volume fraction of monomer would still diffuse from theunexposed region to the exposed region, providing the desired refractiveindex.

The molecular volume of the monomer, however, should not be so large asto slow diffusion below an acceptable rate. Diffusion rates arecontrolled by factors including size of diffusing species, viscosity ofthe medium, and intermolecular interactions. Larger species tend todiffuse more slowly, but it would be possible in some situations tolower the viscosity or make adjustments to the other molecules presentin order to raise diffusion to an acceptable level. Also, in accord withthe discussion herein, it is important to ensure that larger moleculesmaintain compatibility with the matrix.

Numerous architectures are possible for monomers containing multipleindex-contrasting moieties. For example, it is possible for the moietiesto be in the main chain of a linear oligomer, or to be substituentsalong an oligomer chain. Alternatively, it is possible for theindex-contrasting moieties to be the subunits of a branched or dendriticlow molecular weight polymer. The preferred photoactive monomers aredisclosed above.

Typically, 0.1 to 20 wt.% photoactive monomer, based on the weight ofthe photoimageable system, provides desirable results. The preferredacrylate monomers are monofunctional. These include2,4,6-tribromophenylacrylate; 2,4-bis(2-naphthylthio)-2-butylacrylate;pentabromoacrylate; isobornylacrylate; phenylthioethyl acrylate;tetrahydrofurfurylacrylate; i-vinyl-2-pyrrolidinone;2-phenoxyethylacrylate; and the like.

In addition to the photoactive monomer, the optical article typicallycontains a photoinitiator (the photoinitiator and photoactive monomerbeing part of the overall photoimageable system). The photoinitiator,upon exposure to relatively low levels of the recording light,chemically initiates the polymerization of the monomer, avoiding theneed for direct light-induced polymerizatioin of the monomer. Thephotoinitiator generally should offer a source of species that initiatepolymerization of the particular photoactive monomer. Typically, 0.1 to20 wt. % photoinitiator, based on the weight of the photoimageablesystem, provides desirable results.

A variety of photoinitiators known to those skilled in the art andavailable commercially are suitable for use in the invention. It isadvantageous to use a photoinitiator that is sensitive to light in thevisible part of the spectrum, particularly at wavelengths available fromconventional laser sources, e.g., the blue and green lines of Ar+(458,488, 514 nm) and He-Cd lasers (442 nm), the green line of frequencydoubled YAG lasers (532 nm), and the red lines of He-Ne (633 nm) andKr+lasers (647 and 676 nm). One advantageous free radical photoinitiatoris bis(η-5-2,4-cyclopentadien- 1-yl)bis[2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl]titanium, available commercially from Ciba as Irgacure-784.Another visible free-radical photoinitiator (which requires aco-initiator) is 5,7,diiodo-3-butoxy-6-fluorone, commercially availablefrom Spectra Group Limited as H-Nu 470. Free-radical photoinitiators ofdye-hydrogen donor systems are also possible. Examples of suitable dyesinclude eosin, rose bengal, erythrosine, and methylene blue, andsuitable hydrogen donors include tertiary amines such as n-methyldiethanol amine. In the case of cationically polymerizable monomers, acationic photoinitiator is used, such as a sulfonium salt or an iodoniumsalt. These cationic photoinitiator salts absorb predominantly in the UVportion of the spectrum, and are therefore typically sensitized with adye to allow use of the visible portion of the spectrum. An example ofan alternative visible cationic photoinitiator is(η₅-2,4-cyclopentadien-1-yl) (η₆-isopropylbenzene)-iron(II)hexafluorophosphate, available commercial from Ciba as Irgacure 261. Itis also conceivable to use other additives in the photoimageable system,e.g., inert diffusing agents having relatively high or low refractiveindices.

Preferably, the photoinitiators are selected according to theirsensitivity to the light sources. For example, Irgacure 369, Irgacure819, and Irgacure 907 are suitable for commercial blue laser systems.Irgacure-784 is suitable for green laser systems, and CB-650 is suitablefor red laser systems. Irgacure products are available from Ciba; CB-650is available from the Spectra Group.

Advantageously, for holographic recording, the matrix is a polymerformed by isocyanate-hydroxyl step polymerization, more advantageously apolymer formed by isocyanate-hydroxyl step polymerization having apolyether backbone. The polyether backbone offers desirablecompatibility with several useful photoactive monomers, particularlyvinyl aromatic compounds. Specifically, photoactive monomers selectedfrom styrene, bromostyrene, divinyl benzene, and4-methylthio-1-vinylnaphthalene (MTVN) have been found to be useful withmatrix polymers formed by isocyanate-hydroxyl step polymerization andhaving a polyether backbone. A monomer that has more than oneindex-contrasting moiety and that is also useful with these polyethermatrix polymers is 1-(3-(naphth-1-ylthio)propylthio)-4-vinylnaphthalene.Other more preferred monomers include2,4-bis(2-naphthylthio)-2-butylacrylate, and tribromophenyl acrylate.

To be independent, the polymerization reactions for the matrix precursorand the photoactive monomer are selected such that: (a) the reactionsproceed by different types of reaction intermediates, (b) neither theintermediate nor the conditions by which the matrix is polymerized willinduce substantial polymerization of the photoactive monomer functionalgroups, and (c) neither the intermediate nor the conditions by which thematrix is polymerized will induce a non-polymerization reaction of themonomer functional groups that causes cross-reaction (between themonomer functional groups and the matrix polymer) or inhibits laterpolymerization of the monomer functional groups. According to item (a),if a matrix is polymerized by use of an ionic intermediate, it would besuitable to polymerize the photoactive monomer by use of a free radicalreaction. In accordance with item (b), however, the ionic intermediateshould not induce substantial polymerization of the photoactive monomerfunctional groups. Also in accordance with item (b), for example, onemust be aware that a photoinitiated free radical matrix polymerizationwill typically induce a photoinitiated cationic polymerization of aphotoactive monomer functional group. Thus, two otherwise independentreactions are not independent for purposes of the invention if both aredriven by a single reaction condition. In accordance with item (c), forexample, base-catalyzed matrix polymerization should not be performedwhen the photoactive monomer functional group undergoes anon-polymerization reaction in response to the base, even ifpolymerization of the monomer functional group is performed by anindependent reaction. A specific example is that a base-catalyzedepoxy-mercaptan polymerization should not be used with an acrylatemonomer because, although the acrylate is polymerized by a free radicalreaction, the acrylate will react with the mercaptans under basecatalysis, resulting in a cross-reaction.

Table 1 below illustrates some examples of matrix/photoactive monomercombinations where the matrix polymerization reaction and photoactivemonomer polymerization are capable of being independent, and exampleswhere the polymerizations interfere with each other. (Photoactivemonomers are horizontal, and matrix polymers are vertical. “X” indicatescross-reaction or monomer polymerization during matrix polymerization.“O” indicates independent reactions. “I” indicates that the photoactivemonomer polymerization is inhibited by the reagents or reaction thatform the polymeric matrix, e.g., the photoactive monomer functionalgroup is converted to a non-polymerizing group, or chemical species arepresent after the matrix cure that substantially slow the rate or yieldof polymerization of the monomer functional groups.) TABLE 1 (Meth)Styrene Vinyl acrylates Derivatives Ethers Epoxies Cationic Epoxy O O XX Cationic Vinyl O O X X Ethers Epoxy (amine) X O I X Epoxy (mercaptan)X O I X Unsaturated ester X O I X (amine) Unsaturated ester X O I X(mercaptan) Hydrosilylation X X X O Urethane O O O X formation

Thermal stabilizers are commonly incorporated in the compositions toprevent the monomers from premature polymerization by heat that isgenerated by the exotherm from the matrix polymerization reactions. Avariety of thermal stabilizers or inhibitors known to those skilled inthe art and available commercially has been found to have limitedeffectiveness in preventing premature polymerization of the photoactivemonomer for this invention.

For purposes of the invention, polymers are considered to be compatibleif a blend of the polymers is characterized, in 90° light scattering, bya Rayleigh ratio (R_(90°)) less than 7×10⁻³cm⁻¹. The Rayleigh ratio,R_(θ), is a conventionally known property, and is defined as the energyscattered by a unit volume in the direction 0, per steradian, when amedium is illuminated with a unit intensity of unpolarized light, asdiscussed in M. Kerker, The Scattering of Light and OtherElectromagnetic Radiation, Academic Press, San Diego, 1969. The lightsource used for the measurement is generally a laser having a wavelengthin the visible part of the spectrum. Normally, the wavelength intendedfor use in writing holograms is used. The scattering measurements aremade upon a photorecording material that has been flood exposed. Thescattered light is collected at an angle of 90° from the incident light,typically by a photodetector. It is possible to place a narrowbandfilter, centered at the laser wavelength, in front of such aphotodetector to block fluorescent light, although such a step is notrequired. The Rayleigh ratio is typically obtained by comparison to theenergy scatter of a reference material having a known Rayleigh ratio.

Polymer blends which are considered to be miscible, e.g., according toconventional tests such as exhibition of a single glass transitiontemperature, will typically be compatible as well, i.e., miscibility isa subset of compatibility. Standard miscibility guidelines and tablesare therefore useful in selecting a compatible blend. However, it ispossible for polymer blends that are immiscible to be compatibleaccording to the light scattering test above.

A polymer blend is generally considered to be miscible if the blendexhibits a single glass transition temperature, T_(g), as measured byconventional methods. An immiscible blend will typically exhibit twoglass transition temperatures corresponding to the T_(g) values of theindividual polymers. T_(g) testing is most commonly performed bydifferential scanning calorimetry (DSC), which shows the T_(g) as a stepchange in the heat flow (typically the ordinate). The reported T_(g) istypically the temperature at which the ordinate reaches the mid-pointbetween extrapolated baselines before and after the transition. It isalso possible to use Dynamic Mechanical Analysis (DMA) to measure T_(g).DMA measures the storage modulus of a material, which drops severalorders of magnitude in the glass transition region. It is possible incertain cases for the polymers of a blend to have individual T_(g)values that are close to each other. In such cases, conventional methodsfor resolving such overlapping T_(g) should be used, such as discussedin Brinke et al., “The thermal characterization of multi-compohentsystems by enthalpy relaxation,” Thermochimica Acta., 238 (1994), at 75.

Matrix polymer and photopolymer that exhibit miscibility are capable ofbeing selected in several ways. For example, several publishedcompilations of miscible polymers are available, such as 0. Olabisi etal, Polymer-Polymer Miscibility, Academic Press, New York, 1979; L. M.Robeson, MMI, Press Symp. Ser., 2, 177, 1982; L. A. Utracki, PolymerAlloys and Blends: Thermodynamics and Rheology, Hanser Publishers,Munich, 1989; and S. Krause in Polymer Handbook, J. Brandrup and E. H.Immergut, Eds., 3rd Ed., Wiley Interscience, New York, 1989, pp. VI347-370, the disclosures of which are hereby incorporated by reference.Even if a particular polymer of interest is not found in suchreferences, the approach specified allows determination of a compatiblephotorecording material by employing a control sample.

Determination of miscible or compatible blends is further aided byintermolecular interaction considerations that typically drivemiscibility. For example, it is well known that polystyrene andpoly(methylvinylether) are miscible because of an attractive interactionbetween the methyl ether group and the phenyl ring. It is thereforepossible to promote miscibility, or at least compatibility, of twopolymers by using a methyl ether group in one polymer and a phenyl groupin the other polymer. It has also been demonstrated that immisciblepolymers are capable of being made miscible by the incorporation ofappropriate functional groups that can provide ionic interactions. (SeeZ. L. Zhou and A. Eisenberg, J. Polym. Sci., Polym. Phys. Ed., 21 (4),595, 1983; R. Murali and A. Eisenberg, J. Polym. Sci., Part B: Polym.Phys., 26 (7), 1385, 1988; and A Natansohn et al., Makromol. Chem.,Macromol. Symp., 16, 175, 1988). For example polyisoprene andpolystyrene are immiscible. However, when polyisoprene is partiallysulfonated (5%), and 4-vinyl pyridine is copolymerized with thepolystyrene, the blend of these two functionalized polymers is miscible.It is contemplated that the ionic interaction between the sulfonatedgroups and the pyridine group (proton transfer) is the driving forcethat makes this blend miscible. Similarly, polystyrene and poly(ethylacrylate), which are normally immiscible, have been made miscible bylightly sulfonating the polystyrene. (See R. E. Taylor-Smith and R. A.Register, Macromolecules, 26, 2802, 1993.) Charge-transfer has also beenused to make miscible polymers that are otherwise immiscible. Forexample it has been demonstrated that, although poly(methyl acrylate)and poly(methyl methacrylate) are immiscible, blends in which the formeris copolymerized with (N-ethylcarbazol-3-yl)methyl acrylate (electrondonor) and the latter is copolymerized with2-[(3,5-dinitrobenzoyl)oxy]ethyl methacrylate (electron acceptor) aremiscible, provided the right amounts of donor and acceptor are used.(See M.C. Piton and A. Natansohn, Macromolecules, 28, 15, 1995.)Poly(methyl methacrylate) and polystyrene are also capable of being mademiscible using the corresponding donor-acceptor co-monomers (See M. C.Piton and A. Natansohn, Macromolecules, 28, 1605, 1995).

A variety of test methods exist for evaluating the miscibility orcompatibility of polymers, as reflected in the recent overview publishedin A. Hale and H. Bair, Ch. 4-“Polymer Blends and Block Copolymers,”Thermal Characterization of Polymeric Materials, 2nd Ed., AcademicPress, 1997. For example, in the realm of optical methods, opacitytypically indicates a two-phase material, whereas clarity generallyindicates a compatible system. Other methods for evaluating miscibilityinclude neutron scattering, infrared spectroscopy (IR), nuclear magneticresonance (NMR), x-ray scattering and diffraction, fluorescence,Brillouin scattering, melt titration, calorimetry, andchemilluminescence. See, for example, L. Robeson, supra; S. Krause,Chemtracts-Macromol. Chem., 2, 367, 1991a; D. Vessely in Polymer Blendsand Alloys, M. J. Folkes and P. S. Hope, Eds., Blackie Academic andProfessional, Glasgow, pp. 103-125; M. M. Coleman et al. SpecificInteractions and the Miscibility of Polymer Blends, TechnomicPublishing, Lancaster, PA, 1991; A. Garton, Infrared Spectroscopy ofPolymer Blends, Composites and Surfaces, Hanser, New York, 1992; L. W.Kelts et al., Macromolecules, 26, 2941, 1993; and J. L. White and P. A.Mirau, Macromolecules, 26, 3049, 1993; J. L. White and P. A. Mirau,Macromolecules, 27, 1648, 1994; and C. A. Cruz et al., Macromolecules,12, 726, 1979; and C. J. Landry et al., Macromolecules, 26, 35, 1993.

Compatibility has also been promoted in otherwise incompatible polymersby incorporating reactive groups into the polymer matrix, where suchgroups are capable of reacting with the photoactive monomer during theholographic recording step. Some of the photoactive monomer will therebybe grafted onto the matrix during recording. If there are enough ofthese grafts, it is possible to prevent or reduce phase separationduring recording. However, if the refractive index of the grafted moietyand of the monomer is relatively similar, too many grafts, e.g., morethan 30% of monomers grafted to the matrix, will tend to undesirablyreduce refractive index contrast.

A holographic recording medium of the invention is formed by adequatelysupporting the photorecording material, such that holographic writingand reading is possible. Typically, fabrication of the medium involvesdepositing the matrix precursor/photoimageable system mixture betweentwo plates using, for example, a gasket to contain the mixture. Theplates are typically glass, but it is also possible to use othermaterials transparent to the radiation used to write data, e.g., aplastic such as polycaronate or poly(methyl methacrylate). It ispossible to use spacers between the plates to maintain a desiredthickness for the recording medium. During the matrix cure, it ispossible for shrinkage in the material to create stress in the plates,such stress altering the parallelism and/or spacing of the plates andthereby detrimentally affecting the medium's optical properties. Toreduce such effects, it is useful to place the plates in an apparatuscontaining mounts, e.g., vacuum chucks, capable of being adjusted inresponse to changes in parallelism and/or spacing. In such an apparatus,it is possible to monitor the parallelism in real-time by use of aconventional interferometric method, and make any necessary adjustmentsduring the cure. Such a method is discussed, for example, in U.S. patentapplication Ser. No. 08/867,563, the disclosure of which is herebyincorporated by reference. The photorecording material of the inventionis also capable of being supported in other ways. For instance, it isconceivable to dispose the matrix precursor/photoimageable systemmixture into the pores of a substrate, e.g., a nanoporous glass materialsuch as Vycor, prior to matrix cure. More conventional polymerprocessing is also invisioned, e.g., closed mold formation or sheetextrusion. A stratified medium is also contemplated, i.e., a mediumcontaining multiple substrates, e.g., glass, with layers ofphotorecording material disposed between the substrates.

The medium of the invention is then capable of being used in aholographic system such as discussed previously. The amount ofinformation capable of being stored in a holographic medium isproportional to the product of: the refractive index contrast, An, ofthe photorecording material, and the thickness, d, of the photorecordingmaterial. (The refractive index contract, An, is conventionally known,and is defined as the amplitude of the sinusoidal variations in therefractive index of a material in which a plane-wave, volume hologramhas been written. The refractive index varies as: n(x)=n₀+Δn cos(K_(x)),where n(x) is the spatially varying refractive index, x is the positionvector, K is the grating wavevector, and no is the baseline refractiveindex of the medium. See, e.g., P. Hariharan, Optical Holography:Principles, Techniques, and Applications, Cambridge University Press,Cambridge, 1991, at 44.) The An of a material typically calculated fromthe diffraction efficiency or efficiencies of a single volume hologramor a multiplexed set of volume holograms recorded in a medium. The An isassociated with a medium before writing, but is observed by measurementperformed after recording. Advantageously, the photorecording materialof the invention exhibits a Δ of 3×10⁻³ or higher.

Examples of other optical articles include beam filters, beam steerersor deflactors, and optical couplers. (See, e.g., L. Solymar and D.Cooke, Volume Holography and Volume Gratings, Academic Press, 315-327(1981), the disclosure of which is hereby incorporated by reference.) Abeam filter separates part of an incident laser beam that is travelingalong a particular angle from the rest of the beam. Specifically, theBragg selectivity of a thick transmission hologram is able toselectively diffract light along a particular angle of incidence, whilelight along other angle travels undeflected through the hologram. (See,e.g., J. E. Ludman et al., “Very thick holographic nonspatial filteringof laser beams,” Optical Engineering, Vol. 36, No. 6, 1700 (1997), thedisclosure of which is hereby incorporated by reference.) A beam steereris a hologram that deflects light incident at the Bragg angle. Anoptical coupler is typically a combination of beam deflectors that steerlight from a source to a target. These articles, typically referred toas holographic optical elements, are fabricated by imaging a particularoptical interference pattern within a recording medium, as discussedpreviously with respect to data storage. Medium for these holographicoptical elements are capable of being formed by the techniques discussedherein for recording media or waveguides.

As mentioned previously, the material principles discussed herein areapplicable not only to hologram formation, but also to formation ofoptical transmission devices such as waveguides. Polymeric opticalwaveguides are discussed for example in B. L. Booth, “OpticalInterconnection Polymers,” in Polymers for Lightwave and IntegratedOptics, Technology and Applications, L. A. Hornak, ed., Marcel Dekker,Inc. (1992); U.S. Pat. No. 5,292,620; and U.S. Pat. No. 5,219,710, thedisclosures of which are hereby incorporated by reference. Essentially,the recording material of the invention is irradiated in a desiredwaveguide pattern to provide refractive index contrast between thewaveguide pattern and the surrounding (cladding) material. It ispossible for exposure to be performed, for example, by a focused laserlight or by use of a mask with a non-focused light source. Generally, asingle layer is exposed in this manner to provide the waveguide pattern,and additional layers are added to complete the cladding, therebycompleting the waveguide. The process is discussed for example at pages235-36 of Booth, supra, and Cols. 5 and 6 of U.S. Pat. No. 5,292,620. Abenefit of the invention is that by using conventional moldingtechniques, it is possible to mold the matrix/photoimageable systemmixture into a variety of shapes prior to matrix cure. For example, thematrix/photoimageable system mixture can be molded into ridgewaveguides, wherein refractive index patterns are then written into themolded structures. It is thereby possible to easily form structures suchas Bragg gratings. This feature of the invention increases the breadthof applications in which such polymeric waveguides would be useful.

In one embodiment, the present invention comprises the followingingredients: NCO-terminated prepolymers 20-50 Wt % Acrylate Monomers1-15 Wt % Photoinitiators 0.2-3 Wt % Polyols 40-75 Wt % Catalysts 0.1-3Wt % Thermal Stabilizers and Oxidizers 0.001-0.5 Wt %

The NCO-terminated prepolymers are selected from the by-products ofdiols and diisocyanates that have wt % contents of NCO in the range of10 to 25. The NCO contents were determined based on the prepolymer,unreacted diisocyanate and optionally added neat polyisocyanates toachieve the high performance characteristics. Preferred NCO-terminatedprepolymers are the reaction products of polyether polyols withpolyisocyanates. Some commercially available products can be used; forexample, from Bayer Corporation: Baytec WE-series, ME-series, MP-series,and MS-series; from Air Products and Chemicals: the Airthane series.Aliphatic polyisocynates based prepolymers are preferred. However, whenthe NCO-terminated prepolymer is based on aliphatic diisocyanates, 5 to100% of its wt % contents of NCO have to be derived from aromaticdiisocyanates or aliphatic polyisocyanates. Preferred aromaticdiisocyanates are, but no limit to, diphenylmethane diisocyanate (MDI)and toluene diisocyanate (TDI). More preferred are aliphaticpolyisocyanates such as Hexamethylene diisocyanate (HDI) and its biuret,isocyanurate, uretidione derivatives; methylenedi(cyclohexylisocyanate); trimethylhexamethylenediisocyanates; andisophoronediisocyanate.

Preferred photoactive monomers can be represented by the followingchemical structure.

where R can be phenyl, mono or multi-substituted phenyls, bromophenyl,or naphthalene group; R₁ and R₃ can be methylene, ethylene, propylene,or butylene group; R₂ can be H, or alkyl group; R₃ can be COCH═CH₂,COCCH₃═CH₂, or CH═CH₂.

Two acrylate compounds as examples to the above chemical structure are2,4-bis(2-naphthylthio)-2-butylacrylate and1,4-bis(4Bromophenylthio)-2-butylacrylate. The preferred acrylatemonomers are monofunctional. These include 2,4,6-tribromophenylacrylate;pentabromoacrylate; isobornylacrylate; phenylthioethyl acrylatetetrahydrofurfurylacrylate; 2,4-bis(2-naphthylthio)-2-butylacrylate;1-vinyl-2-pyrrolidinone; 1,4-bis(4-bromobenzenethio)-2-butanol;2-phenoxyethylacrylate; and the like. The most preferred acrylatemonomer is 2,4-bis(2-naphthylthio)-2-butylacrylate and1,4-bis(4-bromobenzenethio)-2-butanol.

Preferably, the photoinitiators are selected according to theirsensitivity to the light sources. For example, Irgacure 369, Irgacure819, and Irgacure 907 are suitable for commercial blue laser systems.Irgacure 784 is suitable for green laser systems, and CB-650 is suitablefor red laser systems. Irgacure products are available from Ciba, CB-650is available from the Spectra Group.

Polyols are selected from diols and triols of polytetramethylene glycol,polycaprolactone, polypropylene oxide. Preferred polyols arepolypropylene oxide triols with molecular weight ranging from 450 to6,000. Preferably the polyols are free of moisture contents. Hightemperature vacuum distillation treatments or additives such as moisturescavengers may be used to assure no water residue remains in the polyolsbefore use.

Tin catalysts could be used. These are dimethyltin carboxylate,dimethyltindilaurate, dibutyltindilaurate, stannous octoate, and others.

Additives include thermal stabilizers such as butyrated hydroxytoluene(BHT), tri(N-nitroso-N-phenylhydroxylamine)aluminum salt (NPAL),Phenothiazine, hydroquinone, and methylether of hydroquinone; reducerssuch as peroxides, phosphites, and hydroxyamines; and deformers ordeaerators to eliminate entrapped air bubbles.

It has been found that an aluminum salt compound, used by itself or incombination with other thermal stabilizers, provide the desiredstability at high temperatures and humidity. Surprisingly, this aluminumsalt compound had eliminated the inhibition times while maintained otherdesired high performance characteristics, such as high M/#, highsensitivity and low shrinkage of the media.

Preferably, the aluminum salt compound has the following formula:[X−R−N(NO)O]_(n)M²where, X═H, CH₃, OCH₃, F, Cl, CF₃ or SOCH₃; R is an aliphatic, alicyclicor aromatic group preferably having 1-18 carbon atoms; n=0-5; M ² ishydrogen, a group I to III metal, a group VIIIB metal, or a substitutedor unsubstituted NH4 group. Preferably M² is Al³⁺where n=3, and M² isNH₄ ⁺where n=0.

The most preferred compound is an aluminum salt,tri(N-nitroso-N-phenylhydroxylamine)aluminum salt (NPAL), available fromAlbemarle Corporation.

In particular, NPAL was found to be effective in preventing prematurepolymerization of the photoactive monomer. NPAL can be used by itself orin combination with other thermal stabilizers, for example, butyratedhydroxytoluene (BHT). The preferred amounts are from 0.001 to 0.1 wt. %of the total composition.

The invention will be further clarified by the following examples, whichare intended to be exemplary.

Preparation of Novel Photoactive Compounds

-   I. 2,4-Bis(2-naphthylthio)-2-butylacylate    1. Preparation of 1,4-bis(2-naphthylthio)-2butanol, Compound 1

To a mixture containing potassium t-butoxide (2.69 g, 24 mmol) inacetone (40 ml) stirred at RT was added 2-naphthalenethiol (3.21 g, 20mmol). A red color developed, the solids became dissolved and themixture was stirred for another 15 min. 1,4-Dibromo-2-butanol (2.55 g,11 mmol) was added over 15 min, and a precipitate was formed. After onehour, TLC (40%CH₂Cl₂/hexane) showed a complete disappearance of2-naphthalenethiol. The mixture was filtered, washed with acetone (20ml), and the filtrate was concentrated on rotovap. A brown solidobtained was crystallized from cyclohexane to yield Compound 1 as ayellow powdery solid (3.0 g, 76% yield). Recrystallization from iPrOHyielded pure Compound 1.

2. Preparation of 2,4-Bis(2-naphthalenethiol)-2-butylacrylate, Compound2

63 To a solution containing the intermediate butanol Compound 1 (1.96 g,5 mmol) and triethylamine (0.51 g, 5 mmol) in CH₂Cl₂ (40 ml) stirred at0° C. was added acryloyl chloride (0.46 g, 5 mmol) and the solution wasstirred for one hour. It was then washed with 5% NaHCO₃ (10 ml),deionized water (10 ml), dried over MgSO₄ and solvent removed on rotovapto yield Compound 2 as a yellowish, viscous oil (2.10 g, 94%).

II. 1,4-Bis(4-bromobenzenethiol)-2-butylacrylate

-   1. Preparation of 1,4-bis(4-bromophenylthio)-2-butanol, Compound 3

To a mixture of potassium t-butoxide (4.49 g, 40 mmol) in acetone (75ml) stirred at RT was added 4-bromophenylthiol (7.56 g, 40 mmol) over aperiod of 15 min. Potassium t-butoxide became dissolved. To this mixturewas added 1,4-dibromo-2-butanol (4.64 g, 20 mmol) and the mixture wasstirred for one hour. The precipitate formed was filtered, wash withacetone (25 ml), and solvent removed on rotovap to give1,4-bis(4-bromobenzenethiol)-2-butanol Compound 3 as a yellowish solid.Compound 3 was re-crystallized twice from cyclohexane to yield white,fluffy crystals (4.67 g, 52%).

-   2. Preparation of 1,4-bis(4-broobezenethiol)-2-butylacrylate,    Compound 4

To a solution of the intermediate butanol Compound 3 (4.48 g, 10 mmol)and triethylamine in THF (50 ml) and stirred at 0° C. was added acryloylchloride (0.91 g, 10 mmol). A white precipitate appeared. The solutionwas stirred for one hour. It was filtered, washed with THF (20 ml). Itwas then washed with 5% NaHCO3 (15 ml), water (15 ml), dried (MgSO4),and concentrated over Rotovap to yield 4 as a near colorless oil in aquantitative yield.

Examples and Comparative Examples

To fabricate the high temperature and humidity resistant recordingarticle, the NCO-terminated prepolymer and polyol must first be reactedto form a matrix in which the acrylate monomer, which remains unreacted,will reside.

As the reaction of the NCO-terminated prepolymer and polyol aretwo-component system, the NCO-terminated prepolymer, acrylate monomer,photoinitiator, and thermal stabilizers are predissolved to form ahomogeneous solution before charging into one of the holding tanks of aPosiratio two-component metering, mixing and dispensing machine,available from Liquid Control Corp. The polyol, tin catalyst, and otheradditives are premixed and charged into another holding tank. Each tankis then degassed, adjusting dispensing of materials from the tanks tothe desired amount according to the procedures outlined by LiquidControl.

Precise and accurate mixing of the two components, free of entrapped airbubbles, is carried out by metering the liquid from both tankssimultaneously into a helical element static mixer.

To form a holographic recording article, the desired amount of thewell-mixed solution is dispensed onto the inner surface of the bottomsubstrate held by one of the parallel plate. The upper substrate, whichis held by the other parallel plate, is then brought down to come incontact with the solution and held at a predetermined distance from thebottom plate, according to the procedures described in U.S. Pat.5,932,045 issued Aug. 3, 1999, the disclosure of which is herebyincorporated by reference. The entire set-up is held till the solutionbecomes solidified to assure an optically flat article is produced.

High performance holographic recording articles are characterized by lowshrinkage, dynamic range, and sensitivity. Low shrinkage will assurenon-degradation of the recorded holograms and total fidelity of theholographic data to be recovered. Low shrinkage in the range of lessthan 0.2% is required. The dynamic range of a holographic recordingmedium is typically characterized by the parameter, M/#, a measure ofhow many holograms of a give average diffraction efficiency can bestored in a common volume. The M/# is determined by both the refractiveindex contrast and thickness of a medium. Typical values of M/# are 1.5or better. The photosensitivity is characterized by the total exposuretime required to consume the dynamic range of the media. The sensitivitycan be in the range of 5 to 600 seconds.

Details of the measurements of the recording-induced shrinkage, M/#/ 200μm, and sensitivity are described in detail in Applied Physics Letters,Volume 73, Number 10, p. 1337-1339, 7 Sep. 1998, which is incorporatedherein by reference. Angle-multiplexing a series of plane-wave hologramsinto the recording medium produce these measurements. Afrequency-doubled diode-pumped Nd:YAG laser used for recording andrecovery of the multiplexed holograms was spatially filtered andcollimated by a lens to yield a plane-wave source of light. The lightwas then split into two beams by polarizing beam splitters and half-waveplates and intersected at the sample at an external angle of 44°. Thepower of each beam was 2 mW and the spot diameter was 4 mm. Eachhologram is written with a predetermined exposure time. After recording,the material was allowed to sit in the dark for 20 minutes and thenflood cured with a Xenon lamp filtered to transmit wavelengths longerthan 530 nm.

Comparative Example

This comparative example was prepared and evaluated in accordance withthe procedures described above except using the following ingredients toillustrates the performance characteristics of a commercially availablephotoactive compound, tribromophenylacrylate. This comparative exampleillustrates the decay in M/# measurement of an unstable composition.Component 1, Tank A Baytech WE-180 415.7 gm Tribromophenylacrylate 38.0gm Irgacure 784 8.44 gm BHT 210 mg Component 2, Tank B PolypropyleneOxide Triol 577 gm t-Butylperoxide 310 μl Dibutyltindilaurate 10.2 gmProperties of the articles Shrinkage 0.1% Dynamic range, M/#/200 μm 2.40Sensitivity, seconds to write 80% of the sample 251) Baytech WE-180, available from Bayer, is a 50/50 blend ofbiscyclohexylmethane diisocyanate and a NCO-terminated prepolymer basedon biscyclohexylmethane diisocyanate and polytetramethylene glycol.2) Polypropylene Oxide Triol of 1000 molecular weight.

EXAMPLE 1

Samples of Example 1 were prepared and evaluated in accordance with theprocedures of Comparative Example except using the following ingredientsto illustrates the usefulness of NPAL in prolonging the useful life ofthe composition with the additional benefit of eliminating theinhibition time. Component 1, Tank A Baytech WE-180 415.7 gmTribromophenylacrylate 38.0 gm Irgacure-784 8.44 gm BHT 210 mg NPAL 620mg Component 2, Tank B Polypropylene Oxide Triol 577 gmt-Butylhydroperoxide 310 μl Dibutyltindilaurate 10.2 gm PerformanceShrinkage  0.2% Dynamic range, M/#/200 μm 2.46 Sensitivity, seconds towrite 80% of the sample 47.8 Inhibition Time, sec 0 M/# Retained after 1wk in 60° C./85% RH chamber 90.97% M/# Retained after 2 wk in 60° C./85%RH chamber 69.36% M/# Retained after 4 wk in 60° C./85% RH chamber38.72%EXAMPLE 2

Samples of Example 2 were prepared and evaluated in accordance with theprocedures of Comparative Example except using the following ingredientsto illustrates the usefulness of NPAL. Component 1, Tank A BaytechWE-180 415.7 gm Tribromophenylacrylate 38.0 gm Irgacure-784 8.44 gm BHT2080 mg NPAL 800 mg Component 2, Tank B Polypropylene Oxide Triol 577 gmt-Butylhydroperoxide 310 μl Dibutyltindilaurate 10.2 gm PerformanceShrinkage 0.02%   Dynamic range, M/#/200 μm 2.09 Sensitivity, seconds towrite 80% of the sample 21.6 Inhibition Time, sec 1.17 M/# Retainedafter 1 wk in 60° C./85% RH chamber 87% M/# Retained after 2 wk in 60°C./85% RH chamber 84% M/# Retained after 4 wk in 60° C./85% RH chamber60%

EXAMPLE 3

This example illustrates the properties and performance the novelacrylate photoactive monomer, 2,4-bis(2-naphthylthio)-2-butylacrylate.Component 1, Tank A Baytech WE-180 187.8 gm Desmodur N 3200 125.2 gm2,4-bis(2-naphthylthio)-2- 41.2 gm butylacrylate Irgacure-784 7.8 gm BHT0 NPAL 800 mg Component 2, Tank B Polypropylene Oxide Triol 322.2 gmPTMEG-1000 315.6 t-Butylhydroperoxide 310 μl Dibutyltindilaurate 10.2 gmPerformance Shrinkage  0.21% Dynamic range, M/#/200 μm 5.23 Sensitivity,seconds to write 80% of the sample 29.12 Inhibition Time, sec 0 M/#Retained after 1 wk in 60° C./85% RH chamber 96.76% M/# Retained after 2wk in 60° C./85% RH chamber N/A M/# Retained after 4 wk in 60° C./85% RHchamber N/ADesmodur N-3200 is the biuret derivative of HDI, available from BayerPolypropylene Oxide Triol of 1500 molecular weightPTMEG-1000 is polytetramethyleneglycol diol with 1000 molecular weight

The above description is presented to enable a person skilled in the artto make and use the invention, and is provided in the context of aparticular application and its requirements. Various modifications tothe preferred embodiments will be readily apparent to those skilled inthe art, and the generic principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the invention. Thus, this invention is not intended to belimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein. Thisapplication discloses several numerical range limitations. The numericalranges disclosed inherently support any range within the disclosednumerical ranges even though a precise range limitation is not statedverbatim in the specification because this invention can be practicedthroughout the disclosed numerical ranges. The entire disclosure of thepatents and publications referred in this application are herebyincorporated herein by reference.

1. A photoimageable system comprising an aluminum salt compound and an asymmetric acrylate compound comprising sulfur and aromatic moieties, said asymmetric acrylate compound is liquid at room temperature and said photoimageable system retains at least about 40% of a dynamic range when exposed to 60° C./85%RH for 4 weeks.
 2. The photoimageable system of claim 1, wherein said aluminum salt compound comprises a following formula: [X−R−N(NO)O]_(n)M² wherein, X═H, CH₃, OCH₃, F, Cl, CF₃ or SOCH₃; R is an aliphatic group, an alicyclic group or an aromatic group having 1-18 carbon atoms; n=0-5; M² is hydrogen, a group I to III metal, a group VIIIB metal, a substituted NH₄ group or an unsubstituted NH₄ group.
 3. The photoimageable system of claim 2, wherein M² is Al³⁺ with n=3 or M²is NH₄+with n=0.
 4. The photoimageable system of claim 2, wherein the photoimageable system is a two-chemistry system.
 5. The photoimageable system of claim 2, wherein the photoimageable system forms an optical article.
 6. The photoimageable system of claim 1, wherein the photoimageable system forms a holographic recording medium having a dynamic range of greater than 3 and a shrinkage of less than
 0. 1%.
 7. The photoimageable system of claim 1, wherein the photoimageable system is a two-chemistry system.
 8. The photoimageable system of claim 1, further comprising a matrix precursor.
 9. The photoimageable system of claim 8, wherein said matrix precursor comprises a substance selected from the group consisting of a polyol, an aromatic isocyanate, an aliphatic isocyanate, an aromatic diisocyanate, a hexamethylene diisocyanate, a derivative of hexamethylene diisocyanate and combinations thereof.
 10. The photoimageable system of claim 8, wherein said matrix precursor forms a urethane.
 11. A photoimageable system comprising an aluminum salt compound and a photoactive compound comprising a chemical structure represented by

wherein R is a phenyl group, a mono- or multi-substituted phenyl group, a bromophenyl group or a naphthalene group; R₁ and R₃ are a methylene group, an ethylene group, a propylene group, or a butylene group; R₂ is H or an alkyl group; and R₃ is COCH═CH₂, COCCH₃═CH₂, or CH═CH₂.
 12. The photoimageable system of claim 11, wherein said aluminum salt compound comprises a following formula: [X−R−N(NO)O]_(n)M² wherein, X═H, CH₃, OCH₃, F, Cl, CF₃ or SOCH₃; R is an aliphatic group, an alicyclic group or an aromatic group having 1-18 carbon atoms; n=0-5; M²is hydrogen, a group I to III metal, a group VIIIB metal, a substituted NH₄ group or an unsubstituted NH₄ group.
 13. The photoimageable system of claim 12, wherein M² is Al³⁺ with n=3 or M²is NH₄ ⁺with n=0.
 14. The photoimageable system of claim 11, wherein the photoimageable system forms an optical article.
 15. The photoimageable system of claim 11, wherein the photoimageable system forms a holographic recording medium having a dynamic range of greater than 3 and a shrinkage of less than
 0. 1%.
 16. The photoimageable system of claim 11, wherein the photoimageable system forms a holographic recording medium having a dynamic range of greater than 4 and a shrinkage of less than 0.08%.
 17. The photoimageable system of claim 11, wherein the photoimageable system is a two-chemistry system.
 18. The photoimageable system of claim 11, further comprising a matrix precursor.
 19. The photoimageable system of claim 18, wherein said matrix precursor comprises a substance selected from the group consisting of a polyol, an aromatic isocyanate, an aliphatic isocyanate, an aromatic diisocyanate, a hexamethylene diisocyanate, a derivative of hexamethylene diisocyanate and combinations thereof.
 20. The photoimageable system of claim 18, wherein said matrix precursor forms a urethane.
 21. A method of manufacturing an optical article, comprising: obtaining the photoimageable system of claim 1 and reacting at least some ingredients of the photoimageable system to form said optical article.
 22. The method of claim 21, wherein said reacting comprises a polymerization reaction selected from a group consisting of a urethane formation reaction, an urea formation reaction, cationic epoxy polymerization, cationic vinyl ether polymerization, cationic alkenyl ether polymerization, cationic allyl ether polymerization, cationic ketene acetal polymerization, epoxy-amine step polymerization, epoxy-mercaptan step polymerization, unsaturated ester-amine step polymerization, unsaturated ester-mercaptan step polymerization, a hydrosilylation reaction and combinations thereof.
 23. The method of claim 21, wherein the photoimageable system comprises a matrix precursor, said matrix precursor comprising a substance selected from the group consisting of a polyol, an aromatic isocyanate, an aliphatic isocyanate, an aromatic diisocyanate, a hexamethylene diisocyanate, a derivative of hexamethylene diisocyanate and combinations thereof.
 24. The method of claim 21, wherein the photoimageable system comprises a polyol, said polyol comprising a polyol of a material selected from the group consisting of polypropylene oxide, polytetramethylene ether diol and combinations thereof.
 25. The method of claim 21, wherein the optical article is a holographic recording medium.
 26. A method of manufacturing an optical article, comprising: obtaining the photoimageable system of claim 11 and reacting at least some ingredients of the photoimageable system to form said optical article.
 27. The method of claim 26, wherein said reacting comprises a polymerization reaction selected from a group consisting of a urethane formation reaction, an urea formation reaction, cationic epoxy polymerization, cationic vinyl ether polymerization, cationic alkenyl ether polymerization, cationic allyl ether polymerization, cationic ketene acetal polymerization, epoxy-amine step polymerization, epoxy-mercaptan step polymerization, unsaturated ester-amine step polymerization, unsaturated ester-mercaptan step polymerization, a hydrosilylation reaction and combinations thereof.
 28. The method of claim 26, wherein the photoimageable system comprises a matrix precursor, said matrix precursor comprising a substance selected from the group consisting of a polyol, an aromatic isocyanate, an aliphatic isocyanate, an aromatic diisocyanate, a hexamethylene diisocyanate, a derivative of hexamethylene diisocyanate and combinations thereof.
 29. The method of claim 26, wherein the photoimageable system comprises a polyol, said polyol comprising a polyol of a material selected from the group consisting of polypropylene oxide, polytetramethylene ether diol and combinations thereof.
 30. The method of claim 26, wherein the optical article is a holographic recording medium.
 31. The method of claim 26, wherein said photoimageable system retains at least about 40% of a dynamic range when exposed to 60° C./85% RH for 4 weeks.
 32. The method of claim 26, wherein said photoimageable system retains at least about 60% of a dynamic range when exposed to 60° C./85% RH for 4 weeks.
 33. The method of claim 26, wherein said photoimageable system retains at least about 80% of a dynamic range when exposed to 60° C./85% RH for 4 weeks.
 34. A method recording an interference pattern, comprising: exposing a holographic recording medium comprising the photoimageable system of claim 1 to an interference pattern; recording the interference pattern to the holographic recording medium.
 35. The method of claim 34, wherein the holographic recording medium has a dynamic range of greater than 3 and a shrinkage of less than
 0. 1%.
 36. The method of claim 34, further comprising exposing the recorded interference pattern to a reference beam to produce a signal beam.
 37. A method recording an interference pattern, comprising: exposing a holographic recording medium comprising the photoimageable system of claim 11 to an interference pattern; recording the interference pattern to the holographic recording medium.
 38. The method of claim 37, wherein the holographic recording medium has a dynamic range of greater than 3 and a shrinkage of less than
 0. 1%.
 39. The method of claim 37, further comprising exposing the recorded interference pattern to a reference beam to produce a signal beam. 